The External Tank, also known as the ET, was like the "gas tank" for the space shuttle orbiter. It carried propellants—liquid oxygen and liquid hydrogen—that flowed into the Space Shuttle Main Engines (SSMEs), where they combined and ignited to produce almost one and a half million pounds of thrust to help push the space shuttle to orbit. The external tank also served as the structural support for the whole shuttle stack, with attachment points for the orbiter and booster rockets.

During each shuttle mission, all the propellants in the tank were used up by the SSMEs during the shuttle's trip up to orbit. As the space shuttle orbiter had almost reached the speed it needed to stay in orbit about 8.5 minutes after launch, the empty tank detached about 70 miles (113 kilometers) above the Earth's surface. After detaching, the tank fell back towards Earth on a planned path over the Pacific or Indian Oceans, almost all of it disintegrating in the atmosphere on the way down. The external tank is the only component of the shuttle stack that was not reusable—a new one was constructed for each launch.

Three different types of external tanks were built over the course of the 30-year Space Shuttle Program. The first six space shuttle missions used standard-weight tanks (SWTs), but later, two newer types of tanks were developed—lightweight tanks (LWTs) and super lightweight tanks (SLWTs). All three types looked the same on the outside, but have differences in their internal construction and materials.

The California Science Center's External Tank

The external tank destined for display at the California Science Center is ET-94, the last flight-qualified external tank in existence. This lightweight tank (LWT)—donated to the Science Center by NASA—was ordered to support science missions for space shuttle Columbia. Because construction of super lightweight external tanks had already begun, ET-94 was referred to as a "deferred-build" tank. After Columbia was destroyed on its return back through the atmosphere following STS-107 in 2003, ET-94 was studied extensively to try to assess whether the "deferred-build" tank contributed to the accident in any way. Many pieces of foam were removed from the tank, which is why the tank will need some restoration before being put on display at the California Science Center.

With the addition of ET-94, the California Science Center's Samuel Oschin Air and Space Center will be the only place in the world that people will be able to go to see a complete shuttle stack—orbiter, external tank, and solid rocket boosters—with all real flight hardware in launch configuration.

Fast Facts

Most of the external tank is covered with a 1-inch (2.5cm) thick layer of spray-on foam to protect it from the extreme heat of launch.

Though the liquid hydrogen tank is 2.5 times larger than the liquid oxygen tank, the filled hydrogen tank weighs only a fifth of what the filled oxygen tank does. Liquid oxygen is 16 times heavier than liquid hydrogen.

On the first two shuttle missions, the external tank was coated with white paint. On later missions, tanks weren't painted, which made them 600 pounds lighter.